Effects of Recruiting Maneuvers in Patients with Acute
Respiratory Distress Syndrome Ventilated with Protective
Ventilatory Strategy
Salvatore Grasso, M.D.,* Luciana Mascia, M.D.,† Monica Del Turco, M.D.,‡ Paolo Malacarne, M.D.,‡ Francesco Giunta, M.D.,§ Laurent Brochard, M.D.,储 Arthur S. Slutsky, M.D.,# V. Marco Ranieri, M.D.**
Background:A lung-protective ventilatory strategy with low tidal volume (VT) has been proposed for use in acute respiratory
distress syndrome (ARDS). Alveolar derecruitment may occur during the use of a lung-protective ventilatory strategy and may be prevented by recruiting maneuvers. This study examined the hypothesis that the effectiveness of a recruiting maneuver to improve oxygenation in patients with ARDS would be influ-enced by the elastic properties of the lung and chest wall.
Methods:Twenty-two patients with ARDS were studied during use of the ARDSNet lung-protective ventilatory strategy: VTwas
set at 6 ml/kg predicted body weight and positive end-expira-tory pressure (PEEP) and inspiraend-expira-tory oxygen fraction (FIO2)
were set to obtain an arterial oxygen saturation of 90 –95% and/or an arterial oxygen partial pressure (PaO2) of 60 –
80 mmHg (baseline). Measurements of PaO2/FIO2, static volume–
pressure curve, recruited volume (vertical shift of the volume-pressure curve), and chest wall and lung elastance (EstWand
EstL: esophageal pressure) were obtained on zero
end-expira-tory pressure, at baseline, and at 2 and 20 min after application of a recruiting maneuver (40 cm H2O of continuous positive
airway pressure for 40 s). Cardiac output (transesophageal Doppler) and mean arterial pressure were measured immedi-ately before, during, and immediimmedi-ately after the recruiting ma-neuver. Patients were classified a priori as responders and nonresponders on the basis of the occurrence or nonoccur-rence of a 50% increase in PaO2/FIO2 after the recruiting
maneuver.
Results:Recruiting maneuvers increased PaO2/FIO2by 20ⴞ
3% in nonresponders (n ⴝ 11) and by 175 ⴞ 23% (n ⴝ 11; meanⴞ standard deviation) in responders. On zero
end-expi-ratory pressure, EstL(28.4ⴞ 2.2 vs. 24.2 ⴞ 2.9 cm H2O/l) and
EstW(10.4ⴞ 1.8 vs. 5.6 ⴞ 0.8 cm H2O/l) were higher in
nonre-sponders than in renonre-sponders (P< 0.01). Nonresponders had been ventilated for a longer period of time than responders (7ⴞ 1 vs. 1 ⴞ 0.3 days; P < 0.001). Cardiac output and mean arterial pressure decreased by 31ⴞ 2 and 19 ⴞ 3% in nonre-sponders and by 2ⴞ 1 and 2 ⴞ 1% in responders (P < 0.01).
Conclusions:Application of recruiting maneuvers improves oxygenation only in patients with early ARDS who do not have impairment of chest wall mechanics and with a large potential for recruitment, as indicated by low values of EstL.
TRADITIONAL respiratory support for the acute respira-tory distress syndrome (ARDS) involves the use of rela-tively large (10 –15 ml/kg) tidal volumes (VT) to mini-mize atelectasis and positive end-expiratory pressure (PEEP) to improve arterial oxygenation by means of low inspiratory oxygen fractions (FIO2).1More recently,
lung-protective ventilatory strategies have been proposed2 that are based on the large body of animal data indicating that mechanical ventilation with high VT is associated with pulmonary injury indistinguishable from ARDS.3 Cycling end-expiratory collapse with tidal inflation may exacerbate this process.4Three recent randomized con-trolled trials supported these experimental findings, showing that a lung-protective ventilatory strategies based on low VTis able to decrease markers of pulmo-nary and systemic inflammation5and decrease mortality
among patients with ARDS.6,7
The American–European consensus conference on ARDS proposed periodic use of recruiting maneuvers to prevent atelectasis when small VTand/or low PEEP lev-els are used.8Alveolar derecruitment may occur during
mechanical ventilation with low VT, depending on the FIO2, the regional ventilation/perfusion ratios, and the
end-expiratory lung volume.9,10 On the basis of these recommendations, several studies have investigated the physiologic effects of recruiting maneuvers in patients with ARDS.6,10 –14
Alterations in respiratory mechanics parallel the time course of ARDS.15,16Most patients with early ARDS who have been on the ventilator for only a few days have a static volume pressure (V-P) curve with a marked lower inflection point (LIP) and an upper inflection point (UIP) that occurs well above tidal inflation. By contrast, most patients with late ARDS who have been on the ventilator for several days often have a static V-P curve with an absent LIP and a UIP that occurs within tidal inflation.15,16
Impairment of chest wall mechanics in patients with ARDS has been demonstrated17; recent studies suggest
This article is accompanied by an Editorial View. Please see: Suter PM: Does the advent of (new) low tidal volumes bring the (old) sigh back to the intensive care unit? ANESTHESIOLOGY 2002; 96:783– 4.
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* Clinical attending, Servizio di Anestesiologia e Rianimazione, Ospedale Di Venere. † Doctoral student, Dipartimento di Neuroscienze-Sezione di Fisiolo-gia, Universita’ di Torino. ‡ Clinical attending, § Professor, and ** Associate Professor, Dipartimento di Chirurgia–Terapia Intensiva, Cattedre di Anestesiologia e Rianimazione, Ospedale S. Chiara, Università di Pisa. 储 Professor, Service de Réanimation Medicale, Hopital Henri Mondor, Université Paris XII. # Professor, St. Michael’s Hospital, University of Toronto.
Received from the Servizio di Anestesiologia e Rianimazione, Ospedale Di Venere, Bari, Italy; Dipartimento di Neuroscienze-Sezione di Fisiologia, Universita di Torino, Torino, Italy; Dipartimento di Chirurgia–Terapia Intensiva, Cattedre di Anestesiologia e Rianimazione, Ospedale S. Chiara, Università di Pisa, Pisa, Italy; Service de Réanimation Medicale, Hopital Henri Mondor, Université Paris XII, Paris, France; and St. Michael’s Hospital, University of Toronto, Toronto, Canada. Submitted for publication July 6, 2001. Accepted for publication October 18, 2001. Supported by Consiglio Nazionale delle Ricerche (grant No. 95.00934) and Ministero Universita e Ricerca Scientifica e Tecnologica (MURST)-2001, Rome, Italy. Drs. Grasso and Mascia equally contributed to the study and should therefore both be considered as first authors.
Address reprint requests to Dr. Ranieri: Universita Di Torino, Sezione Di Anestesiologia e Rianinazione Ospedale S. Giovanni Battista, Corso Dogliotti 19, 10126, Torino, Italy. Address electronic mail to: marco.ranieri@unito.it. Individ-ual article reprints may be purchased through the Journal Web site, www.anesthesiology.org.
that alterations of chest wall mechanics may influence the effects of PEEP on arterial oxygenation18 and respi-ratory mechanics.19
The current study set out to examine the hypothesis that the effectiveness of the recruiting maneuver to im-prove oxygenation in patients with ARDS could be in-fluenced by the elastic properties of the lung and chest wall.
Methods
Patient Selection
Twenty-two patients with ARDS were recruited from the intensive care units (ICUs) of the Di Venere, Poli-clinico (University of Bari), and S. Chiara (University of Pisa) hospitals. The review boards of all three hospitals approved the research protocol, and informed consent was obtained from all patients or next of kin. Inclusion criteria were age ⬎18 yr and diagnosis of ARDS.20 Ex-clusion criteria were cardiogenic pulmonary edema (clinically suspected or pulmonary artery occlusion pres-sure ⬎18 mmHg), history of ventricular fibrillation or tachyarrhythmia, unstable angina or myocardial infarc-tion within the preceding month, preexisting chronic obstructive pulmonary disease, mean arterial pressure
(MAP) ⬍65 mmHg (despite attempts to increase blood
pressure with fluid and vasopressors, as clinically indi-cated), anatomic chest wall abnormalities, chest tube with persistent air leak, pregnancy, and intracranial abnormality.
All patients were intubated and ventilated with a Sie-mens Servo Ventilator 300 (SieSie-mens Elema AB, Solna, Sweden). The investigation was performed in the
semirecumbent position after sedation (diazepam,
0.1– 0.2 mg/kg, and fentanyl, 2–3 g/kg) and paralysis (vecuronium, 4 – 8 mg). A physician not involved in the research aspects of the study was always present to provide patient care.
Study Protocol
Before enrollment, patients were ventilated according to the ARDSNet protective ventilatory strategy: VTwas set at 6 ml/kg predicted body weight; PEEP and FIO2
were set to obtain an arterial oxygen saturation (SaO2)
value of 90 –95% or an arterial oxygen partial pressure (PaO2) of 60 – 80 mmHg (baseline),7or both.
At baseline, static inspiratory V-P curves of the respi-ratory system, chest wall, and lung were obtained with and without PEEP. A recruiting maneuver was then per-formed by setting the ventilator on the continuos posi-tive airway pressure mode and applying a pressure of 40 cm H2O for 40 s.6After termination of the recruiting maneuver, the previous breathing pattern was reestab-lished, and measurements of PaO2/FIO2ratio, V-P curve,
and respiratory mechanics were obtained 2 min and 20 min after application of the recruiting maneuver.
Measurements
Flow was measured with a heated pneumotachograph (Fleisch no. 2; Fleisch, Lausanne, Switzerland), con-nected to a differential pressure transducer (Diff-Cap, ⫾1 cm H2O; Special Instruments, Nordlingen, Germany)
inserted between the Y-piece of the ventilator circuit and the endotracheal tube. The pneumotachograph was linear over the experimental range of flow. Airway open-ing pressure (Pao) was measured proximal to the endo-tracheal tube with a pressure transducer (Digima-Clic,
⫾100 cm H2O; Special Instruments). Changes in
in-trathoracic pressure were evaluated by assessment of esophageal pressure (Pes). Esophageal pressure was measured with a thin latex balloon-tipped catheter con-nected by a polyethylene catheter to a pressure trans-ducer (Digima-Clic,⫾100 cm H2O). The esophageal bal-loon was filled with 1–1.5 ml of air and correctly positioned by means of an occlusion test performed before sedation and paralysis.17–19Transpulmonary pres-sure was calculated as Pao-Pes. All the variables de-scribed above were displayed and collected on a
per-sonal computer through a 12-bit analog-to-digital
converter board (DAQCard 700; National Instrument, Austin, TX) at a sample rate of 200 Hz (ICU Lab, KleisTEK Engineering, Bari, Italy). Arterial blood samples were analyzed (ABL 330; Radiometer, Copenhagen, Denmark).
The difference between end-expiratory lung volume during mechanical ventilation and the elastic equilib-rium volume of the respiratory system on zero end-expiratory pressure (ZEEP) was assessed by reducing respiratory rate to the lowest value possible during a baseline breath, while decreasing PEEP to zero.21 To standardize volume history, immediately after the pro-longed expiration to ZEEP, five consecutive pressure control breaths with an inspiratory pressure of 40 cm H2O and an inspiratory time of 5 s were applied before reestablishing the baseline ventilatory pattern.11,14,22 To-tal PEEP (PEEPtot⫽ applied PEEP plus intrinsic PEEP) of the respiratory system (rs) and of the chest wall (W) were measured as the plateau pressure in Pao and Pes during an end-expiratory occlusion, referenced to their values at the elastic equilibrium point of the respiratory system. PEEPtotapplied to the lung (L) was evaluated as PEEPtot,L⫽ PEEPtot,rs⫺ PEEPtot,W.17
Static inflation V-P curves were obtained by perform-ing sperform-ingle-breath occlusions at different inflatperform-ing vol-umes, achieved by changing inflation volumes in random order, and altering the respiratory frequency of the ven-tilator.21 Twelve to 15 experimental points were col-lected.21Each occlusion was maintained until an appar-ent plateau in Pao was observed (3– 4 s). The static end-inspiratory pressures of the respiratory system (Pstrs) and chest wall (PstW) were measured as the end-inspiratory plateau pressure on Pao and Pes, referenced to their values at the elastic equilibrium volume of the
respiratory system. The static end-inspiratory pressure of the lung (PstL) was calculated as the difference between Pstrs and Pstw. Values of pressures at the upper and lower inflection points of the V-P curve of the lung on ZEEP (UIPL and LIPL, respectively) were quantified by means of a step-by-step regression analysis on samples of 4 –5 consecutive experimental points, as previously de-scribed.22Recruited volume at baseline and 2 and 20 min after application of the recruiting maneuver was identi-fied as the upward shift of the V-P curves of the lung, relative to the curve on ZEEP at a fixed pressure (20 cm H2O).21
Static elastance (Est) of the respiratory system was calculated as Estrs ⫽ (Pstrs ⫺ PEEPcocrs)/VT. Static elas-tance of the chest wall (EstW) was calculated as EstW⫽ (Pstw⫺ PEEPcocw)/VT. Static elastance of the lung (EstL) was calculated as Estrs⫺ EstW.
All patients had a radial artery and central venous catheters for measurements of systemic blood pressure and right atrial pressure. To evaluate the instantaneous effects of the recruiting maneuver on cardiac output,
transesophageal continuous-wave Doppler
(Doptek-ODM1; Deltex Medical, Chichester, UK) of the descend-ing aorta was measured before and durdescend-ing application of the recruiting maneuver (20 –25 s after the onset of the maneuver) and immediately (within 20 –25 s) after rees-tablishment of baseline ventilation. This technique mea-sures blood flow velocity in the descending thoracic aorta, with use of a transducer inserted in the esopha-gus.23Stroke volume may then be derived with an algo-rithm based on (1) the beat-to-beat maximum velocity– time integral (stroke distance); (2) the cross-sectional area of the descending aorta; and (3) a correction factor that transforms descending aortic blood flow into global cardiac output.23 The validity of this approach in me-chanically ventilated critically ill patients has recently been established.23
Patients were defined a priori as responders if they had an increase in PaO2/FIO2ofⱕ50% 2 min after
appli-cation of the recruiting maneuver; otherwise, they were considered to be nonresponders.11 On the basis of
his-tory, clinical presentation, and microbiologic
re-sults,11,19ARDS was classified as pulmonary or extrapul-monary by three independent physicians blinded to the study results.
Statistics
Data are expressed as mean ⫾ standard deviation of the mean. Data within groups were compared by analy-sis of variance for repeated measures with a Bonferroni correction. If significant (Pⱖ 0.05), the values at differ-ent experimdiffer-ental conditions were compared with those at baseline with use of a paired t test, as modified by Dunnett. Comparisons of data between groups were performed at each experimental condition by the Fisher exact test for categorical variables and the Wilcoxon test
for continuous variables. Regression analysis was per-formed with the least-squares method. Analysis was car-ried out with the StatView software package (Abacus, Berkeley, CA).
Results
Two minutes after the application of the recruiting maneuver, PaO2/FIO2 increased 20 ⫾ 3% in 11
nonre-sponders and 175⫾ 23% in the responders (11 patients). Before application of the recruiting maneuver, VT (6.1⫾ 0.1 and 6.0 ⫾ 0.2 ml/kg) and PEEP (9.4 ⫾ 2.2 and
9.1 ⫾ 2.7 cm H2O) did not differ between
nonre-sponders and renonre-sponders. Age (47⫾ 13 [nonresponders] and 42⫾ 18 yr [responders]), sex (five and six males), underlying disease (five and six cases of pulmonary ARDS), and PaO2/FIO2ratio on ZEEP (111⫾ 38 vs. 105 ⫾
38) were similar for nonresponders and responders. Time on mechanical ventilation (including time on ven-tilator in other ICUs before admission to the study cen-ters) was significantly longer (P⬍ 0.001) for the nonre-sponders (table 1).
Values of LIP (8.7⫾ 1.2 vs. 10.6 ⫾ 1.0 cm H2O) and UIP (24.2⫾ 1.4 vs. 27.8 ⫾ 2.2 cm H2O) on the static V-P
Table 1. Demographic and Clinical Characteristics in Nonresponders and Responders
Patient Age Gender
Underlying Disease Time on MV (days) Nonresponders 4 67 M Pancreatitis 10 6 38 M Polytrauma 6 7 61 F Peritonitis 7 8 52 F Pneumonia 6 10 28 M Pneumonia 7 11 49 F Peritonitis 6 14 49 F Peritonitis 7 16 63 M Pneumonia 10 18 45 F Peritonitis 5 21 40 F Pneumonia 7 22 30 M Pneumonia 9 Mean 47 7.1 SD 13 1.5 Responders 1 63 F Pancreatitis 1 2 19 M Polytrauma 1 3 35 M Polytrauma 1 5 25 M Pneumonia 1 9 37 F Pancreatitis 1 12 41 F Pneumonia 1 13 22 F Pneumonia 1 15 33 F Pancreatitis 1 17 68 M Pneumonia 1 19 62 F Pneumonia 1 20 62 M Pneumonia 2 Mean 42 1.0* SD 18 0.3
Data are mean⫾ SD.
* P⬍ 0.001, Wilcoxon for unpaired data Nonresponders vs. Responders. MV⫽ mechanical ventilation; M ⫽ male; F ⫽ female.
curve of the lung on ZEEP were lower for nonresponders than responders (P⬍ 0.01); values of EstL(28.4⫾ 2.2 vs. 24.2⫾ 2.9 cm H2O/l) and EstW(10.4⫾ 1.8 vs. 5.6 ⫾ 0.8 cm H2O/l) on ZEEP were higher for nonresponders than
responders (P⬍ 0.01) (table 2).
Two minutes after the application of the recruiting maneuver, the PaO2/FIO2ratio increased to 180⫾ 46 in
nonresponders and to 440 ⫾ 60 in responders (P ⬍
0.001); 20 min after application of the recruiting maneu-ver, values of PaO2/FIO2tended to return toward baseline
values in both groups (fig. 1, top). Values of EstL(25.1⫾ 2.2 vs. 18.9⫾ 2.4 cm H2O/l) at baseline were higher in
nonresponders than responders (P ⬍ 0.01). Two
min-utes after application of the recruiting maneuver, EstL
decreased to 22.7 ⫾ 1.9 cm H2O/l in nonresponders
(8 ⫾ 3%) and to 14.8 ⫾ 2.3 cm H2O/l in responders
(21 ⫾ 2%) (P ⬍ 0.01). EstL returned toward baseline values 20 min after application of the recruiting maneu-ver in both groups (fig. 1, middle). At baseline, the amount of recruited volume with PEEP was smaller
in nonresponders than in responders (199 ⫾ 80 vs.
284⫾ 37 ml, respectively; P ⬍ 0.01) and increased after application of the recruiting maneuver to 296⫾ 99 ml in
nonresponders and 482 ⫾ 80 ml in responders (P ⬍
0.01). Twenty minutes after application of the recruiting
maneuver, the recruited volume was 263 ⫾ 99 ml in
nonresponders and 322 ⫾ 64 ml in responders (P ⬍
0.01; fig. 1, bottom).
Physiologic variables in one representative nonre-sponder and renonre-sponder are shown in figure 2. Relative to baseline conditions, application of the recruiting maneu-ver increased the end-expiratory position of the volume and Pes signals only in the responder. At similar VT, tidal swings of Pes were larger for nonresponders than for responders. Transpulmonary pressure during the sus-tained inflation was lower in nonresponders than in responders. The reduction in blood pressure and the increase in right atrial pressure after application of the recruiting maneuver were more evident in nonre-sponders than renonre-sponders.
On average, heart rate remained unchanged during application of the recruiting maneuver (20 –25 s after onset), and cardiac output, stroke volume, and MAP
decreased by 31 ⫾ 2, 27 ⫾ 1, and 19 ⫾ 3% in
nonre-sponders and by 2⫾ 1, 5 ⫾ 1, and 2 ⫾ 1% in responders, whereas right atrial pressure increased by 19 ⫾ 3 and 2⫾ 1%, respectively (P ⬍ 0.01). All hemodynamic vari-ables returned to baseline values right after ventilation was reestablished after the recruiting maneuver (within 20 –30 s; table 3).
Discussion
All ventilatory strategies used to minimize alveolar re-cruitment– derecruitment and overdistension based on
mechanical properties use Pstrs as a surrogate for
transpulmonary pressure. However, the lung and the chest wall are in series, and Pstrsequals the sum of PstL and PstW. Our study shows that a substantial part of the pressure applied to the respiratory system during a recruit-ment maneuver (to reexpand collapsed alveoli) can be dissipated against a stiff chest wall; we found that the pressure applied to the lung during a fixed recruitment maneuver of 40 cm H2O was 18.4⫾ 3.3 and 28.6 ⫾ 2.1 cm H2O in nonresponders and responders, respectively.
Matamis et al.15 found that alterations in respiratory mechanics paralleled the evolution of ARDS. In patients on the ventilator for a prolonged period of time and with signs of interstitial fibrosis on a chest radiograph, static V-P curves differed substantially from those observed in patients at an early stage and at the onset of mechanical ventilation. Our study included patients transferred from other ICUs and on mechanical ventilation for 5–10 days (nonresponders) and patients admitted from the emer-gency department or from the ward and on mechanical ventilation for 1–2 days (responders). Patients in whom
Table 2. Respiratory Mechanics in Nonresponders and Responders on Zero End-expiratory Pressure
Patient
LIPL UIPL Est,L Est,W
(cm H2O) (cm H2O) (cm H2O/L) (cm H2O/L) Nonresponders 4 8.1 22.8 33.64 9.90 6 9.1 23.5 29.06 10.70 7 10.8 25.1 25.11 11.60 8 6.9 24.5 30.23 9.30 10 8.7 22.9 27.50 11.50 11 9.5 26.7 26.00 9.70 14 8.4 22.7 31.00 10.90 16 7.6 22.9 28.50 9.30 18 7.8 24.2 27.90 6.40 21 10.2 26.4 26.80 12.90 22 8.5 23.5 31.80 11.90 Mean 8.7 24.2 28.4 10.4 SD 1.2 1.4 2.2 1.8 Responders 1 10.1 27.5 25.67 5.30 2 9.8 26.6 23.55 6.60 3 10.7 24.3 28.82 5.20 5 9.2 26.3 27.92 6.80 9 10.8 28.8 23.10 5.50 12 11.5 31.1 27.20 4.30 13 12.5 25.7 24.50 5.50 15 10.3 26.9 22.40 6.10 17 11.8 28.5 19.90 6.20 19 10.4 30.8 21.90 5.80 20 9.8 29.8 20.80 4.30 Mean 10.6* 27.8* 24.2* 5.6* SD 1.0 2.2 2.9 0.8
Data are mean⫾ SD.
* P⬍ 0.01, Wilcoxon for unpaired data Nonresponders vs. Responders. LIPL⫽ lower inflection point on the static pressure-volume curve of the lung;
UIPL⫽ upper inflection point on the static pressure-volume curve of the lung;
application of a recruiting maneuver caused a substantial improvement in oxygenation were those studied within 1–2 days after initiation of mechanical ventilation. In these patients, EstL on ZEEP was smaller and LIPL and UIPLoccurred at higher pressures than in patients stud-ied after 5–10 days of mechanical ventilation. This sug-gests that the potential for alveolar recruitment with recruiting maneuvers may be reduced in patients with late ARDS.
Recent studies have demonstrated impaired chest wall mechanics in many patients with ARDS consequent to
major abdominal surgery17 and in patients in whom
ARDS is caused by extrapulmonary causes.19These stud-ies suggested that a great part of the alteration in chest wall mechanics can be explained by abdominal disten-sion.17,19It has been shown that in patients with ARDS, chest wall mechanics can be significantly altered by the presence of pleural effusions due to a positive fluid balance.11,24 –27Mattison et al.26found that pleural effu-sions were seen only in patients on the ventilator for 7⫾ 1 days, not in patients on the ventilator for only 2⫾ 1 days. We found a positive correlation (R2⫽ 0.72; P ⬍ 0.0001) between EstWon ZEEP and days on mechanical ventilation, suggesting that impairment of chest wall mechanics may occur, independently from the
underly-ing disease, because of pleural effusions in patients on the ventilator for a prolonged period of time. Further studies are required to prospectively confirm this hypothesis.
Pelosi et al.11 showed that recruiting maneuvers
im-proved lung mechanics and oxygenation only in patients with extrapulmonary ARDS. In our study, the underlying disease responsible for ARDS did not influence the amount of improvement in arterial oxygenation after application of the recruiting maneuver. The relation be-tween baseline VT and effects of PEEP and recruiting
maneuvers on alveolar recruitment may explain such apparently conflicting data. In the study of Pelosi et al.,11 VTand Pstrsduring baseline ventilation were 0.56⫾ 0.11
L (approximately 10 ml/kg) and 31.6 ⫾ 3.6 cm H2O,
whereas in the present study they were 0.38⫾ 0.05 L (6.1⫾ 0.1 ml/kg) and 23.3 ⫾ 2.8 cm H2O, respectively. Several studies have shown that lung mechanics vary considerably with volume history.28 –30When relatively large VT(10 –12 ml/kg) are used, most alveolar recruit-ment may occur during tidal inflation, and the potential for further recruitment with PEEP or recruiting maneu-vers may be limited.31This is confirmed by the observa-tion that alveolar recruitment with PEEP decreases with increasing magnitude of Pstrson ZEEP.21,32,33The larger
Fig. 1. Individual values of arterial oxygen partial pressure or inspiratory oxygen frac-tion (PaO2/FIO2; top) ratio static elastance of the lung (EstL; middle) and recruited volume
(bottom) during the different experimental conditions in nonresponders and respond-ers (RMⴝ recruiting maneuver; horizontal bars indicate mean values; *P< 0.01 [analy-sis of variance for repeated measures with Bonferroni correction vs. baseline]; †P < 0.05; #P < 0.01 [Wilcoxon test, nonre-sponders vs. renonre-sponders]).
potential for alveolar recruitment due to the lower VT used in the current study could therefore explain the improvement in arterial oxygenation with recruiting ma-neuvers that was also noted in responders with pulmo-nary ARDS.
Concerns have been voiced about the potential risk of hemodynamic impairment during application of recruit-ing maneuvers.34Our data show that in nonresponders, application of a recruiting maneuver caused a substantial (20 –30%) reduction in MAP and cardiac output. The effects of recruiting maneuvers on MAP and cardiac output include a reduced preload due to transmission of Pao to intrathoracic vasculature and/or an increased af-terload due to increased lung volume.35,36 In patients
with a stiff chest wall, the degree of Pao transmitted to the pleural space would be larger than in patients with a normal chest wall35,36; thus, the decrease in the pressure gradient for venous return (19 ⫾ 3% increase in right atrial pressure) observed in nonresponders during appli-cation of recruiting maneuvers might explain the reduc-tion in cardiac output. The more compliant chest wall observed in the responders may induce a smaller trans-mission of pressure within the thorax, with a larger amount of pressure transmitted to the lung. The smaller decrease in the pressure gradient for venous return (2⫾ 1% increase in right atrial pressure) may explain the minimal hemodynamic consequences due to recruiting maneuvers observed in responders.
Table 3. Hemodynamic Variables before, during, and Immediately after Application of a Recruiting Maneuver
Nonresponders Responders
Before RM During RM After RM Before During RM After RM
HR (beats/min) 95⫾ 10 103⫾ 10 97⫾ 7 93⫾ 11 98⫾ 10 95⫾ 9
CO (L䡠 min⫺1) 10.1⫾ 0.5 6.1⫾ 0.7* 9.45⫾ 0.8 10.6⫾ 1.7 10.5⫾ 1.6 10.8⫾ 1.8
SV (ml) 116⫾ 15 58⫾ 13* 104⫾ 12 118⫾ 19 110⫾ 15 116⫾ 20
MAP (mmHg) 85⫾ 10 70⫾ 5* 86⫾ 6 90⫾ 6 88⫾ 7 91⫾ 8
RAP (mmHg) 15.2⫾ 1.5 24⫾ 2.9* 14.7⫾ 0.9 14.2⫾ 3.3 16.1⫾ 2.7 14.5⫾ 5.1
* P⬍ 0.01, analysis of variance (ANOVA) for repeated measures with Bonferroni’s correction versus before RM. Data are mean⫾ SD.
RM⫽ recruiting maneuver; HR ⫽ heart rate; CO ⫽ cardiac output; SV ⫽ stroke volume; MAP ⫽ mean arterial pressure; RAP ⫽ right atrial pressure. Fig. 2. Physiologic variables in a represen-tative nonresponder and responder be-fore, during, and after application of a re-cruiting maneuver. From top to bottom: flow, airway opening pressure (Pao), and changes in lung volume (⌬V), esophageal pressure (⌬Pes), transpulmonary pressure (PL), arterial pressure (ABP), and right
However, cyclic right ventricle afterload occurring during the inspiratory phase has been demonstrated in mechanically ventilated patients.37 In a recent study, Vieillard-Baron et al.38 found that when EstW was in-creased by chest strapping and transpulmonary pressure was reduced by decreasing VTwithout changing Pao, the right ventricle was unloaded. In our study the increase in lung volume and transpulmonary pressure during the recruiting maneuver was smaller in nonresponders with worsening hemodynamics (fig. 2). Under these circum-stances, the increase in right ventricle afterload would not likely be the mechanism responsible for the reduc-tion in cardiac output and MAP observed during the application of recruiting maneuvers in nonresponders. Echocardiographic evaluation of hemodynamics, not performed in the present study, may confirm these speculations.
One may argue that application of a higher level of continuous positive airway pressure for a longer period of time may have transformed nonresponders into re-sponders. However, the reduction in cardiac output and MAP observed in nonresponders suggests first that the use of more aggressive recruiting maneuvers may worsen the hemodynamic impairment and therefore limit the clinical use of recruiting maneuvers at a pres-sures higher than 40 cm H2O, and second, in patients with late ARDS, characterized by a focal distribution of loss of aeration,39 recruiting maneuvers may provide alveolar recruitment with lung overdistension.40Loss of beneficial effects of the recruiting maneuver was ob-served within 30 min. This may be the result of an insufficient level of PEEP to keep open the alveoli re-cruited by sustained inflation.34
In conclusion, this study demonstrates that application of recruiting maneuvers is successful in improving oxy-genation only in patients with early ARDS on the venti-lator for 1–2 days and without impairment of chest wall mechanics. In patients ventilated for a longer period of time, the presence of a stiff chest wall and the reduction in blood pressure and cardiac output make the recruiting maneuver ineffective and potentially harmful.
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